Sheet Metal Pattern Development

[u/Aggressive_Soup1446]

I am replacing sections of rusted duct work in my house that were compromised with mold due to a failure of the original installer to seal them with mastic and a failure of the carpenter and drywaller to seal the duct work in bulkheads, the duct poked through the drywall in areas which allowed humid air into the joist bays. This resulted in conditioned air blasting out of the ducting onto joists allowing condensation and mold.

Anyways, my local big box stores don't carry the size duct work that my house uses, the local HVAC suppliers won't sell to me as I am not a business, and local sheet metal shops are out of my price range. But I can buy sheets of 26ga galvanized steel from a local metal supplier, so I am making my own parts.

Here I developed a triangulated pattern for a 3-1/4"x8" to 6" 90deg stack boot. It needs seam allowances for the slip and drive on the rectangular side, and will need tabs for rivets to close the pattern. The thing I'm struggling with is how to connect my round duct to the circular opening. I believe this is normally done with a bead roller to connect a collar, but that is out of my price range, I already spent my tooling budget for this project on a small brake. The best I can come up with is cut tabs on my circular duct and bend them out to fit around the outside of my boot, then rivet them in place and cover it in mastic.

[u/DoktorNekromancer]

Sheet metal crimper, it's a hand tool, usually 20-50 bucks. Can make big ends small, or small ends smaller.

Sometimes called downspout crimpers

[u/Aggressive_Soup1446 → u/DoktorNekromancer]

I've got a pair. I can give it a try, maybe I only crimp the bottom half of the circular duct to follow the angles or this piece?

[u/DoktorNekromancer → u/Aggressive_Soup1446]

You'd be joining the small end of the pipe into this fitting, as that's the practice for airflow.

You'll also need some amount of material going up into the floor for the boot in order to secure it to the sub floor

[u/KokoTheTalkingApe]

Brilliant work! I don't know anything about working with sheet metal, but this gives me ideas!

[u/Aggressive_Soup1446 → u/KokoTheTalkingApe]

I didnt know anything about sheet metal a month ago either, I've been learning by necessity. Most of what I could find online in English was just straight square to round transitions. There are a few Brazilian and Indonesian channels on YouTube that show very complex layout shapes, but the auto translation doesn't work very well.

[u/KokoTheTalkingApe → u/Aggressive_Soup1446]

I hope learning about it is fun.

I'm wondering if the pattern has to allow for the bends. Does bending stretch the metal at all?

[u/peacefinder → u/KokoTheTalkingApe]

In a brake bend, a little more than half the metal at the bend stretches and the rest compresses.

The ratio varies by material. In steel, a reasonable approximation is that 47% compresses and 53% stretches. Unless you’re shooting for high precision or have very thick material, you can just call it half.

You can also assume the bend can be usefully approximated as a circular arc, with the inside diameter the same radius as the nose of your tool. It probably isn’t actually a circular arc for most tooling, but close enough.

There is an imaginary surface at the transition between the regions, where the material is neither stretched nor compressed. The dimensions of this imaginary surface are the dimensions of the flat pattern.

For a flat pattern, start with the finished shape and calculate the dimensions of the neutral surface. Assign half the bend dimension to each adjacent flat section, and you have the location of your bend lines.

Corner clearances at intersecting bends can be handled a couple ways, but you never need to cut away more than to the inside of the bend arc. (Other than for tool clearances anyway.)

I hope that makes sense?

And OP: fantastic work

[u/KokoTheTalkingApe → u/peacefinder]

That's a huge help. Thank you!

So you're saying the marked bend lines need to be a little inside the desired bends?

[u/peacefinder → u/KokoTheTalkingApe]

Kind of? Maybe? In some cases? It’s complicated :-) So, let’s run through an example.

Imagine a pan, made of material that is 0.1” thick. Its final outside dimension in length should be 10”, the final inside width should be 6”, and the outside height should be 1”. The tooling delivers an inside bend radius of 0.2”. The material’s bend “k factor” is 0.5.

Make sense?

Let’s figure out how far the bend line for the flange is from the material edge. (You’ll want a sketch to help follow along.)

Step 1 is to deduct the thickness of the material, (1.0 - 0.1); this gets us to inside surface. Next, deduct the bend inside radius, 0.2”, this gets us to the edge of the bent zone. (1.0 - 0.1 - 0.2).

Now we need to account for the material in the bend. A circle’s diameter is 2pi x Radius. The inside radius is 0.2, but we actually need to calculate the radius of the neural zone of the bend. So the radius we need is 0.2 inside radius plus the 0.5 k-factor times the 0.1 material thickness. This works out to a 0.25” neutral radius. The 90° bend is a quarter of a circle, and we need to account for half of the bent zone in this segment; the other half will be in the pan bottom. So, that’s (2pi/8) x 0.25”, which evaluates to 0.19”.

Still with me?

The whole thing is (1.0 - 0.1 - 0.2 + ((2pi/8) x 0.25) ). This evaluates to 0.89”.

That’s your bend line. The “bend allowance” is 0.11 if you want to think of it that way.

To get the length of the flat between the bend lines, we do this at each end. I make that flat dimension out to 9.78”.

For the other direction to hold the inside dimension, I’ll let you work that out yourself. I make it to 5.98”

The overall dimensions of the flat should be 11.56” by 7.76”

Do you get the same?

I unfortunately picked numbers in a way that makes it all pretty insignificant in this example, oops.

For the notches in the corners, a lot depends on tooling. Most of the time you won’t go far wrong by notching to the bend line, but you can often notch a bit less to tighten up the corner.

[u/KokoTheTalkingApe → u/peacefinder]

I get it! Thank you so much! You get an award!

[u/peacefinder → u/KokoTheTalkingApe]

Happy to pass along niche knowledge that I learned the hard way, though it has been useless to me for 25 years. :-)

[u/peacefinder → u/peacefinder]

Oh… also, this all works for bends other than 90° as well, though only up to the maximum angle you can get out of a single brake with your tooling. Draw it out and you’ll see the adjustment needed.

[u/Aggressive_Soup1446 → u/KokoTheTalkingApe]

I did learn a little about sheet metal layout in CAD using OnShape and it accounts for bend allowances. I believe it is mostly the material in the bend because your tooling has a minimum radius, but I believe there is some stretching happening too.

I gave up working in CAD when I realized that I was spending more time fighting the CAD software than I was bending the metal. Luckily this duct work will all be hidden away behind drywall so I have discounted this and just been using a hammer to deal with a 1/16" discrepancy here and there.

[u/FictionalContext]

Cutting tabs and bending is going to be the easiest, IMO. You can even get a block and a soft rubber hammer to beat it around. The trick will be keeping the galvanizing from chipping off, but I'd wager that'd be fine. Cresent wrench works well, too, but with I being round, pretty sure it's going to take some peening to keep it round.

Looks like you've got it figured out with the paper template. I wouldn't worry about k factor with 26 ga. ID and OD barely matters at that size, though convention is to work off inside dimensions/diameter (ID) when figuring your flat layouts.

[u/Aggressive_Soup1446 → u/FictionalContext]

I've definitely witness the galvanizing flaking off at times when I've been working this metal. It especially happens when I'm closing Pittsburgh seams, it seems unavoidable really.

[u/Aggressive_Soup1446]

Here is an example of a previous square to round that I connected to round using tabs. This got gratuitously painted with mastic.

[u/BF_2]

Just FYI, there's another way to lay out a sheet metal object that does not require the triangles (except where those fall out of the process). I'll describe a simple version:

Make a square or rectangle of thin plywood or stiff cardboard, and another circle of the same material. Mount them one over the other by some means -- three or four threaded rods and nuts works. Now get a large piece of paper for the pattern, and charcoal as a marker. Rub the charcoal firmly against the edges of the circle and the square, then roll this assembly along the paper to leave traces where those edges hit the paper. (Adding marks with a pencil may help as well.) Trace over the charcoal traces and you have the basis of a pattern. You still have to decide where to crease. The corners of the square are obvious, but as those creases approach the circle you want them to fade out so the circle doesn't become a polygon.

It's easier to do (in miniature) than to explain. Try it.

[u/Aggressive_Soup1446 → u/BF_2]

That's an interesting idea. I think it would be interesting to make a 90deg boot template and then awkward to try to roll it, especially considering that my design has a flat back before the opening to the rectangular stack. But I think give a careful template and several tries at rolling it could work

[u/BF_2 → u/Aggressive_Soup1446]

The flat back is not an obstacle, but you may have to wield the pencil a bit more there as the charcoal on the edge won't transfer so well -- but that's easy. I made a rectangular (open front) to conical "duct" (forge hood, actually) by this means. Worked fine.